1. Field of the Invention
The present invention relates to a liquid crystal display device, and to a method of fabricating a liquid crystal display device, particularly a color filter on thin film transistor (COT) type liquid crystal display (LCD) device and a method of fabricating the same.
2. Discussion of the Related Art
In general, liquid crystal display (LCD) devices make use of optical anisotropy and polarization properties of liquid crystal molecules to produce images. When an electric field is applied to liquid crystal molecules, the liquid crystal molecules are rearranged. As a result, the transmittance of the liquid crystal molecules is changed according to the alignment direction of the rearranged liquid crystal molecules.
The LCD device includes two substrates disposed with their respective electrodes facing each other, and a liquid crystal layer is interposed between the respective electrodes. When a voltage is applied to the electrodes, an electric field is generated between the electrodes to modulate the light transmittance of the liquid crystal layer by rearranging liquid crystal molecules, thereby displaying images.
FIG. 1 is an exploded perspective view of a liquid crystal display panel according to the related art. As shown in FIG. 1, a liquid crystal panel 11 includes an upper substrate 5, a lower substrate 22 and a liquid crystal material 14 interposed between the upper and lower substrates 5 and 22. A black matrix 6 is formed on the upper substrate 5 and a color filter layer 8 including sub-color filters is formed on the black matrix 6. A common electrode 18 is formed on the color filter layer 8. A pixel electrode 17 and a thin film transistor (TFT) “T” used as a switching element are formed on the lower substrate 22 in a pixel region “P.” The pixel electrode 17 is formed of a transparent conductive material, such as indium-tin-oxide (ITO) and indium-zinc-oxide (IZO). The pixel region “P” is defined by a gate line 13 and a data line 15 and the TFT “T” disposed in matrix is connected to the gate line 13 and the data line 15.
A storage capacitor “C” is connected in parallel to the pixel electrode 17 and formed over the gate line 13. A portion of the gate line 13 is used as a first electrode of the storage capacitor “C”, and a metal pattern 30 with an island shape, which is in the same layer and is the same material as the source and drain electrodes of the TFT “T,” is used as a second electrode of the storage capacitor “C.” Because the metal pattern 30 is connected to the pixel electrode 17, the same signal is applied to the metal pattern 30 and the pixel electrode 17.
The upper substrate 5 and the lower substrate 22 may be referred to as a color filter substrate and an array substrate, respectively. Although not shown in FIG. 1, the liquid crystal panel 11 may be embedded between top case and a bottom case to constitute a liquid crystal display (LCD) device.
FIG. 2 is a schematic cross-sectional view of an LCD device according to the related art. In FIG. 2, a liquid crystal panel “D” includes a first substrate 40, a second substrate 70 and liquid crystal (not shown) interposed between the first and second substrates 40 and 70. A polarizing film 80 is formed on an outer surface of the second substrate 70 and the liquid crystal panel “D” is fixed using a top case “TC.” A thin film transistor (TFT) “T” including a gate electrode 42, an active layer 50, a source electrode 58 and a drain electrode 60 is formed on the first substrate 40. In addition, a gate line 14 and a data line (not shown) crossing each other to define a pixel region “P” are formed on the first substrate 40. A gate pad 46 is formed on one end of the gate line 14 and a gate pad terminal 66 of a transparent conductive material is formed on the gate pad 46. Although not shown in FIG. 3, a data pad is formed at one end of the data line with data pad terminal is formed on the data pad. The gate pad terminal 66 and the data pad terminal are connected to an external circuit (not shown).
The first substrate 40 and the second substrate 70 are attached to each other using a sealant 84. An alignment key “K” to attach the first substrate 40 and the second substrate 70. An edge portion of the second substrate 70 having the alignment key is cut out after attaching the first and second substrates 40 and 70 to expose the gate pad terminal 66 and the data pad terminal in the edge portion on the second substrate 70. A light-shielding pattern 74 is formed on a first portion of the second substrate 70 corresponding to the sealant 84. A black matrix 72 is formed on a second portion of the second substrate 70 corresponding to the TFT “T,” the gate line 44 and the data line.
In general, a liquid crystal device includes a liquid crystal panel, a backlight unit, a top case and a bottom case. The top case is attached to the bottom case with the liquid crystal panel and the backlight unit within the two cases. The liquid crystal panel has a display area and a non-display area at the periphery of the display area. The display area is exposed through the top case and the non-display area is covered by the top case. However, the top case does not completely cover the non-display area of the liquid crystal panel. Accordingly, an additional light-shielding pattern is required in the non-display area of the liquid crystal panel.
The alignment key “K”, the light shielding pattern 74 and the black matrix 72 are simultaneously formed using a first mask process on the second substrate 70. A color filter layer 76 is formed on a third portion of the second substrate 70 corresponding to the pixel region “P” using a second mask process on the second substrate 70. A common electrode 78 is formed over the entire surface of the second substrate 70 having the black matrix 72 and the color filter layer 76. Further, a patterned spacer 82 of a transparent organic material is formed on the common electrode 78 corresponding to the TFT “T” using a third mask process on the second substrate 70.
Because the liquid crystal panel “D” is obtained by attaching the lower substrate 40 having array elements such as the gate line 44, the data line and the TFT “T and the second substrate 70 having the black matrix 72 and the color filter layer 76, the liquid crystal panel “D” may deteriorate due to a light leakage resulting from an alignment error. In order to overcome these problems, a color filter on TFT (COT) type has been suggested where a color filter layer is formed on the first substrate having a TFT.
FIG. 3 is a schematic cross-sectional view of a COT type LCD device according to the related art. In FIG. 3, a COT type liquid crystal panel 100 includes a first substrate 110 and a second substrate 150 attached to each other with a sealant 180. A TFT “T” having a gate electrode 112, an active layer 120, a source electrode 122 and a drain electrode 124 is formed on the first substrate 110. In addition, a gate line 114 and a data line (not shown) crossing each other to define a pixel region “P” are formed on the first substrate 110. A gate pad 116 is formed on one end of the gate line 114 and a data pad (not shown) is formed at one end of the data line. A color filter layer 128 including red, green and blue sub-color filters (128a, 128b and not shown) and a black matrix 130 are formed on the first substrate 110 having the TFT “T.” The color filter layer 128 corresponds to the pixel region “P” and the black matrix 130 corresponds to a channel region “CH” of the TFT “T.” A transparent pixel electrode 134 contacting the drain electrode 124 is formed on the color filter layer 128 and a first orientation film 136 of polyimide is formed on the pixel electrode 134. A common electrode 152 is formed on the second substrate 150 and a second orientation film 154 is formed on the common electrode 152. In addition, a patterned spacer 158 is formed on the second orientation film 154 corresponding to the TFT “T.”
In the COT type liquid crystal panel 100, the first and second orientation films 136 and 154 are formed so as not to contact the sealant 180 because the polyimide for the orientation films has a poor contact property with the sealant 180. When one of the first and second orientation films 136 and 154 contact the sealant 180, the sealant 180 may be broken. Accordingly, the first and second orientation films 136 and 154 are spaced apart from the sealant 180 by a predetermined distance “S1” as a fabrication margin. Since the predetermined distance “S1” causes an increase in the use of liquid crystal materials and fabrication costs, it is necessary to reduce the predetermined distance “S1.” In the COT type liquid crystal panel 100, the color filter layer 128 and the black matrix 130 are formed on the first substrate 110 such that an additional deposition, photolithographic and etch steps are required to make an alignment key on the second substrate 150. Moreover, since the black matrix 130 is formed on the first substrate 110, additional deposition, photolithographic and etch steps are required to form a light-shielding pattern, which prevents leakage light at the boundary of the display area of the liquid crystal panel on the second substrate 150. Thus, in the COT-type LCD device, additional steps are required to form an alignment key and a light-shielding pattern on a second substrate, and these additional steps increase fabrication costs and fabrication time.